Project Summary: Research on efficacious adolescent substance abuse interventions over the past two decades has consistently demonstrated heterogeneity of response to treatment. Key challenges for the .field include understanding these various response trajectories and establishing strategies to adapt treatments to enhance their effectiveness for those who are not responsive to standard interventions. Such research is critically needed to inform treatment services providers on optimal care strategies. An important advance in treatment services research efforts is the increasing movement away from a "one-size-fits-all" perspective inherent in standardized interventions and a call for the development of adaptive interventions that incorporate treatment algorithms to aid clinical decision-making. The Sequential Multiple Assignment Randomized Trial (SMART) outlines a process whereby a series of randomizations within an individual study is used to create an optimal adaptive intervention. We propose two studies to examine the efficacy of eight potential adaptive treatment strategies using the SMART design. Study 1 goals are twofold (1) to examine which of two distinct sets of criteria for defining early treatment response (abstinence vs. minimal use) is the best guide for subsequent treatment decisions, i.e., how best to determine when response to a minimal treatment intervention is "good enough" for an adolescent to step out of treatment without risking a significant relapse. (2) to determine which of two stepped adaptive interventions result in better outcomes for treatment non-responders and relapsers: an enhanced version of the same treatment or a new treatment? Participants (n=80) will receive an initial dose of treatment consisting of MET/CBT8. At completion, participants will be randomly assigned, in sequential steps, to one of two treatment response criteria and, if needed, one of two stepped interventions. Pre- to post-treatment outcomes will be examined using the SMART protocol framework to identify the specific clinical components comprising the optimal adaptive treatment strategy in response to a particular treatment outcome. Study 2 is a pilot (n=60) test of the efficacy of the optimal adaptive strategy identified in Study 1 vs. a standard MET/CBT 12 group intervention, with follow up assessments at 3 and 6 months post-treatment. Relevance: This research will provide a systematic set of optimal decision rules for treatment agencies on how best to proceed when adolescents receiving an initial course of MET/CBT8 show initial treatment response, persist in their substance use, or demonstrate an initial treatment response followed by later relapse. This type of "practical" clinical trial begins the long overdue process of addressing critical gaps between traditional treatment efficacy research and key questions that emerge in clinical practice such as determining when adolescents have received sufficient treatment, how to proceed when initial treatment efforts are not effective, and how best to address relapse. Mechanisms of change will also be examined in Study 2.